This invention relates to a one- or two-component polyurethane lacquer for coating elastomers, to its production and to its use.
There are a number of industrial applications, including for example the sealing of window compartments for moving glass parts in automobiles, which require elastomers having improved surface slip with respect to glass in addition to their other well-known favourable properties. To achieve this, it has hitherto been possible to coat the elastomer part with a lacquer containing fixedly joined spacers of hard materials. The spacers used include particles of glass, aluminium, polypropylene or polyvinyl chloride. A corresponding process is described, for example, in DE-C 33 35 150.
A basic disadvantage is that the peak-and-trough structure of the spacer-containing lacquer does not provide for optimal sealing so that head winds can penetrate and cause noise which is undesirable. Another disadvantage is that the surface of the elastomer parts thus lacquered does not form an optically smooth glossy surface, but instead looks like fine emery cloth.
Another process described in U.S. Pat. No. 4,572,875 and in U.S. Pat. No. 4,572,871 comprises adding silicones or fluorinated resins to the anti-friction lacquer. Corresponding lacquers have the disadvantage that, under extreme weather conditions, such as heat, cold, wind and rain, to which the elastomer part is normally exposed, the added silicones and fluorinated resins are removed from the lacquer which leads to a reduction in the surface slip of the elastomer part with respect to glass and is undesirable. A further disadvantage is that the silicones are oils which are known to act as release agents.
In view of their characteristic properties, these release agents are carrier over relatively easily and can give rise to difficulties, for example at the assembly stage in the bonding and painting of bodywork parts. Accordingly, the use and consumption of non-fixed silicone oils is something which automobile manufacturers regard as undesirable.
As described in EP-A 0 293 084, silicone oils can be fixed by selecting reactive polysiloxanes which "react in" substantially irreversibly. However, the anti-friction properties of corresponding lacquers are still in need of improvement. An automobile glass window provided with an integrally moulded frame of a polymer is known from DE-OS 42 27 935 corresponding to EP-A-0 585 167. It is distinguished by the fact that the surface parts of the frame intended for contact with the window frame of the bodywork are coated with a soft-look two-component polyurethane lacquer which preferably consists of a polyol component of a solvent-containing aliphatic, hydroxyfunctional polyester and an isocyanate component of an aromatic isocyanate. In a particularly preferred embodiment, an automobile glass window is coated with a soft coating lacquer containing as its polyol component an aliphatic polyester with a solvent mixture of butyl acetate, toluene and xylene to which titanium dioxide powder and iron oxide powder are added as inorganic pigments. The isocyanate component of this lacquer is an aromatic isocyanate in a solvent mixture of toluene and xylene, the two components mentioned--mixed in a ratio of 8:1--being applied in such a way that a dry layer thickness of around 80 micrometers is formed.
One-component polyurethane lacquers have also been known to the expert on elastomers for several years, cf. for example G. Glement in Kautschuk und Gummi, Kunststoffe, Vol. 23 (1970), No. 8, pages 375-378.
Thus, EP-A-0 251 334 describes a composition prepared by reactions of hydroxyterminated perfluoropolyesters, silicocarbinols, diisocyanates and polyols in a solvent. The molar ratios are selected so that these reactive components form an NCO-terminated prepolymer which, applied as a film, cures under the effect of moisture. All four components are reacted at the same time.
EP-B 0 375 923 describes moisture-curing one-component polyurethane lacquers for coating elastomers which contain polyurethane prepolymers with--on average--two or more isocyanate groups per molecule prepared by mixing alcohols having a functionality of two or higher with an excess of isocyanate compounds having a functionality of two or higher and solvent, reactive polysiloxanes and reactive surfactants being used to improve surface slip. The surfactants used are compounds containing a perfluorinated C.sub.6-12 alkyl radical and a non-fluorinated hydrophilic group containing a group reactive to isocyanate-terminated lacquer constituents. The lacquers in question may advantageously contain further auxiliaries, for example retarders, accelerators, stabilizers and pigments. However, they may only be applied in the form of very thin films. Moreover, it is apparent from the Examples that the films obtained only have a coefficient of friction of 0.25 to 0.4
The problem addressed by the present invention was to provide one- or two-component polyurethane lacquers for coating elastomers which would have a lower coefficient of friction than the moisture-curing one-component polyurethane lacquers for coating elastomers known from EP-A-0 375 923. In addition, the one- or two-component polyurethane lacquer according to the invention would also have other performance advantages over the prior art, i.e. high abrasion resistance and very high resistance to moisture and UV light.